1. Field of the Invention
The embodiment discussed herein is related to a manufacturing method of a silicon carbide semiconductor device.
2. Description of the Related Art
Among semiconductor devices conventionally used as power devices, those using silicon (Si) as a semiconductor material have become mainstream. Compared to silicon, silicon carbide (SiC) has a wider bandgap (hereinafter, wide gap semiconductor) and physical properties such as thermal conductivity that is 3 times that of silicon, critical electric field strength that is 10 times that of silicon, and electron drift velocity that is 2 times that of silicon. Therefore, the application of SiC for power devices capable of high temperature operation with low loss and for which the dielectric breakdown voltage is high, is being researched.
A vertical semiconductor device having a back surface electrode equipped with a low resistance ohmic electrode in the back surface side is a mainstream power device structure. Various materials and structures are used for the back surface electrode of a vertical semiconductor device. For instance, surface electrodes have been proposed that are respectively formed by stacked layers including a titanium (Ti) layer, a nickel (Ni) layer, and a silver (Ag) layer (for example, refer to Japanese Laid-Open Patent Publication No. 2007-184571); and a titanium layer, a nickel layer, and a gold layer (for example, refer to Japanese Laid-Open Patent Publication No. 2010-86999).
As a method of forming a back surface electrode of a vertical semiconductor device that uses SiC, a method has been proposed where a nickel layer is formed on a semiconductor substrate of silicon carbide (hereinafter, SiC substrate) and thereafter, the nickel layer is subjected to heat treatment to form a nickel silicide layer, whereby contact (electrical contact unit) of the SiC substrate and the nickel silicide layer is an ohmic contact (for example, refer to Japanese Laid-Open Patent Publication Nos. 2007-184571 and 2010-86999). Nonetheless, in Japanese Laid-Open Patent Publication Nos. 2007-184571 and 2010-86999, a problem arises in that the back surface electrode formed on the nickel silicide layer is prone to peeling from the nickel silicide layer.
To resolve such a problem, a method has been proposed where after the nickel layer remaining on the surface of the nickel silicide layer has been removed exposing the nickel silicide layer, the back surface electrode is formed by sequentially stacking on the nickel silicide layer, the titanium layer, the nickel layer, and the gold layer (for example, refer to Japanese Laid-Open Patent Publication No. 2008-53291). In Japanese Laid-Open Patent Publication No. 2008-53291, by forming the portion contacting the nickel silicide layer of the back surface electrode by a metal other than nickel, peeling of the back surface electrode can be suppressed. Further, even if a layer of deposited carbon (C) is formed on a surface of the nickel silicide layer, the layer including the carbon can be removed together with the nickel layer whereby, peeling of the back surface electrode is suppressed.
Further, as another method of forming a back surface electrode of a semiconductor device, a method has been proposed where residual metal carbide or carbon particles on the surface of the nickel silicide layer are removed by plasma etching and thereafter, a back surface electrode is formed on the nickel silicide layer (for example, refer to Japanese Laid-Open Patent Publication No. 2003-243323). Further, as a method of planarizing a surface of a semiconductor wafer, a method has been proposed that includes a machine processing process of machine processing a (000-1)C face of silicon carbide and a surface layer removing process of removing 0.1 μm or more of the surface layer of the silicon carbide by thermal oxidation using a substance containing oxygen atoms after the machine processing process (for example, refer to Japanese Patent No. 4539140).
Further, as a method of planarizing a surface of a semiconductor wafer, a method has been proposed where, a colloidal silica slurry including sodium hypochlorite and hydrogen peroxide solution, and having an oxidation-reduction potential at least 700 mV or more with respect to a standard hydrogen electrode together with a dissolved oxygen concentration of 20 mg/L or more is used as an oxidizing agent, the formed oxidized layer of the surface of the silicon carbide single crystal wafer is mechanically polished using a polishing surface pressure of 0.05 kg/cm2 or greater and 0.20 kg/cm2 or less, and removed (for example, refer to Japanese Patent No. 4846445). In Japanese Patent No. 4846445, final polishing is performed after the surface SiC is oxidized to change the SiC to SiOx having substantially the same hardness as colloidal silica.
Nonetheless, even when the techniques described in Japanese Patent Application Laid-Open Publication No. 2008-53291 or Japanese Patent Application Laid-Open Publication No. 2003-243323 are used to form a back surface electrode, a problem arises in that the adhesion of the nickel silicide layer and the titanium layer, which is the lowest layer of the back surface electrode, is low. For example, when the semiconductor wafer is diced into chips, the back surface electrode has been confirmed to peel from the nickel silicide layer. The reason for this is presumed to be as follows. According to Japanese Laid-Open Patent Publication No. 2007-184571, the nickel silicide layer is generated by a solid-phase reaction of silicon carbide and nickel, indicated below as equation (1).Ni+2SiC→NiSi2+2C  (1)
Carbon resulting from the reaction of equation (1) is present dispersed throughout the nickel silicide layer as very fine deposits or in an over-saturated state in which the crystals are unstable. When heat treatment is performed after the nickel silicide layer is formed, the carbon dispersed through the nickel silicide layer is discharged all at once and is deposited (agglomerates) in a layer as deposits, such as graphite, on the surface of and inside the nickel silicide layer. These deposits of agglomerated carbon are brittle and have low adhesion; and consequently, even with the slightest stress, easily fracture, whereby the back surface electrode formed on the nickel silicide layer peels.
In a manufacturing process of a semiconductor device using SiC, after the nickel silicide layer is formed, various types of heat treatment are performed whereby, carbon of the SiC substrate diffuses and is deposited inside and on the surface of the nickel silicide layer. The carbon that is deposited on the surface of the nickel silicide layer can be removed by a process such as ion milling, enabling peeling of the back surface electrode consequent to the deposition of carbon on the surface of the nickel silicide layer to be prevented. However, removal of the carbon that is deposited inside the nickel silicide layer is extremely difficult and a problem arises in that the back surface electrode peels from portions where carbon has been deposited in a layer inside the nickel silicide layer.